Before electronic battery feed circuits existed, a hybrid transformer was used to convert a balanced two wire signal from an unbalanced four wire signal. In the last few years there has been a considerable amount of effort, however, to replace the hybrid transformer due to its size, weight and cost. With the advent of electronic battery feed circuits, there is a further need to reconfigure the hybrid circuit so that it will be suitable for newer electronic battery feed circuit designs. Present state-of-the-art does not permit an inexpensive solution to this problem, primarily due to the high voltages involved. The -50 volt office battery, which is used to provide power, exceeds the voltage rating of most semiconductor devices on the market today.
The problem of designing a circuit to withstand the higher voltages of an office battery has generally been approached in one of three ways. First, a semiconductor integrated circuit which uses a manufacturing process that can operate at higher voltages can be designed. This is an expensive solution, however. Second, an intermediate power supply, with a voltage of about half of the office battery, can be added to the office power supply. This, however, necessitates additional equipment, power buses, wire routing and connectors. Third, one can derive an intermediate, secondary voltage from the -50 v battery by using Zener diodes, or some other voltage regulating device. Higher power consumption will result from this technique, however.
The present invention falls into the third category of deriving a secondary voltage from the -50 volt battery. Since operational amplifiers are not normally capable of handling the battery voltages found in a central office, the two operational amplifiers are placed in series, and two Zener diodes are used to provide a lower stable supply voltage which the operational amplifiers can safely use. Using a -50 volt battery, such a circuit configuration will consume about 170 milliwatts of power.